Moving server.h/cpp for two reasons:

1. There is no server class in server.h, so the name is inaccurate.
2. I want to create an RPCServer class and use server.h as a plain header

without all of this existing garbage in it.

I know the "helpers" name is kind of lame, but it will need to be split up into separate
header files in the near future as the RPC refactor continues.

Depends on D1583.

make check